FAX and TIFF

1. FAX and TIFF • Modified Huffman coding is used in fax machines. • TIFF also uses this technique. • It combines the variable length codes of Huffman coding with the coding of repetitive data in run length encoding. 4. 1

2. FAXes • Faxes have only white bits and black bits. • Each line is coded as a series of alternating runs of white and black bits. • Runs of 63 or less are coded with a terminating code. • Runs of 64 or greater require that a makeup code prefix the terminating code. • The makeup codes are used to describe runs in multiples of 64 from 64 to 2560. 4. 2

3. Terminating codewords 4. 3

4. Makeup Codewords 4. 4

5. White and Black pixels • Studies have shown that most facsimiles are 85 percent white. • Faxes assume that any line begins with a run of white bits. • If it does not, a run of white bits of 0 length must begin the encoded line. 4. 5

6. Error Recovering • Each scan line ends with a special EOL (end of line) character consisting of eleven zeros and a 1 (000000000001). • There is no code that has more than seven leading zeroes. • There is no code that has more than three ending zeros. 4. 6

7. Error Recovering (Cont.) • If the decoder sees eleven zeros, it will recognize the end of line and continue scanning for a 1. • Upon receiving the 1, it will then start a new line. • If a bit in a scanned line gets corrupted, the most that will be lost is the rest of the line. • If the EOL code gets corrupted, the most that will get lost is the next line. 4. 7

8. Example 0 white 00110101 1 black 010 4 white 1011 2 black 11 1 white 0111 1 black 010 1266 white 011011000 + 01010011 EOL 000000000001 4. 8